Large-amplitude conformational changes in proteins, including loop motions, relative motions between domains, collective breathing of protein cores, ligand-binding or oligomerization reactions, and overall folding-unfolding events, may be closely coupled, and in some instances rate-limiting, to biological functions such as molecular recognition, transitions along the catalytic cycle of enzymes, and inhibition or activation of proteins through intra- or inter-molecular protein-protein interactions. Mutations that perturb dynamical processes and conformational equilibria are associated with significant pathology, including loss or gain of function and misfolding. Recent developments, including those from the PI laboratory, have opened new opportunities for investigation of large amplitude conformational dynamic processes on microsecond-millisecond time scales using NMR spin relaxation measurements at equilibrium in solution and with atomic site resolution, without potential complications introduced by non-native modifications necessary for other solution-state spectroscopic techniques. Cadherins are calcium-dependent cell-adhesion molecules expressed in most vertebrate tissues. Vertebrate genomes contain some twenty classical (type I and type II) cadherins that contribute to maintenance of tissue integrity and to developmental processes, such as morphogenesis, tissue specification, and neuronal circuit patterning. Cadherin proteins from opposing cells engage in specific homophilic trans interactions mediated by the N-terminal extracellular domain 1 (EC1). The EC1-EC1 dimer interface is formed by -strand exchange or swapping, which leads to the insertion of one (Trp2, Type I cadherins) or two (Trp2 and Trp4, Type II cadherins) tryptophan side chain indole groups, respectively, into the hydrophobic core of the adhesive partner. The proposed research has two primary aims: (1) identification of the differential mechanistic bases for cell adhesion mediated by domain (strand) swapping in the Type I and Type II cadherin superfamily and (2) development of novel experimental and theoretical methods for characterizing protein dynamics on s-ms time scales. Time-dependent structural changes underlie the normal function of proteins, and misfunction in genetic diseases, cancer, and other pathologies; the proposed research will quantify this linkage for the cadherin superfamily of proteins involved in essential cell-cell interactions. Completion of these goals will enable additional future applications to a wide range of protein systems of biological importance.